The Polya-Gamma sampler source files with names beginning stLMM_pg_ are
adapted from BayesLogit.

BayesLogit copyright:

  (C) Nicholas Polson, James Scott, Jesse Windle, 2012-2019

BayesLogit is distributed under the GNU General Public License, version 3 or
later. stLMM is also distributed under GPL-3-or-later.